The present invention relates to an ignition control device for use in the internal combustion engine.
FIG. 1 is a circuit diagram showing the conventional ignition device employed in the internal combustion engine. This prior art ignition device includes a signal generator 2 for generating a rotation speed signals. That signal which represents the rotation speed of a shaft, is picked up by a pickup coil of magnet induction type, for example, an ignition timing detector 4 which generates an output pulse representing the ignition timing responsive to an output signal applied from the signal generator 2. The prior art ignition device also includes a series circuit of a primary winding 6-1 of an ignition coil 6 connected between a power source V.sub.D and the ground, a transistor switching circuit 8 and a resistor 10, and a secondary winding 6-2 of ignition coil 6 and an ignition plug 12 connected in series between the power source V.sub.D and ground.
The prior art ignition device further includes a duty cycle control circuit 14 for generating a duty cycle signal responsive to an output signal applied from the ignition timing detector 4 for determining the period for which the switching circuit 8 is kept ON or OFF, and a circuit 16 for driving the switching circuit 8 responsive to an output signal applied from the duty cycle control circuit 14.
A series circuit of resistors 18 and 20 is further connected in parallel with the resistor 10, and an input terminal of a constant current control circuit 22 is connected to the junction between these resistors 18 and 20. This constant current control circuit detects that the current flowing to the primary winding 6-1 of the ignition coil 6 reaches a predetermined value, controls a driver circuit 16 responsive to this current to keep constant the current flowing to the switching circuit 18, and supplies to the duty control circuit 14 an output signal used to achieve duty control in a succeeding cycle.
With ignition devices of this type, it is required to generate a voltage having a level corresponding to the rotation speed of engine in order to cause the duty cycle control circuit 14 to generate a duty cycle signal. This voltage having a level corresponding to the rotation speed of engine is obtained in the prior art by integrating an output signal which is generated from the timing detector 4 synchronizing with the rotation of shaft. Hence, the response speed for a change in the rotation speed is slow. Further, in the case of integrating the output signal applied from the timing detector 4, ripple component is mixed with the integrated voltage, thus making it difficult to obtain a voltage accurately corresponding to the rotation speed of engine. Furthermore, it is necessary that a reference voltage whose level changes according to the rotation speed of engine is compared with a saw-tooth wave signal generated in synchronism with the rotation movement of engine shaft to control the duty cycle, and it therefore becomes necessary to provide a different circuit for generating a saw-tooth wave signal.